1. Field of the Invention
The present invention relates to antennas and antenna modules that are used in communication using electromagnetic field coupling such as RFID communication.
2. Description of the Related Art
Currently, short-range communication systems, in which a variety of non-contact ICs are included, are widely used in a variety of fields. This type of communication system includes a non-contact IC card, which is equipped with, for example, a wireless communication IC, and a card reader, and communication is performed by bringing the non-contact IC card and the card reader within a predetermined distance from each other. An antenna is needed to perform communication and the resonant frequency of this antenna is set on the basis of the frequency of a communication signal. Examples of such an antenna are described in Japanese Unexamined Patent Application Publication No. 2001-84463 and Japanese Unexamined Patent Application Publication No. 10-334203 and these antennas include a coil electrode, which is wound to have a substantially planar shape, and a structure that causes a capacitance to be generated, which, along with the inductance of the coil electrode, is used to set the resonant frequency.
For example, in Japanese Unexamined Patent Application Publication No. 2001-84463, a coil electrode is provided that is wound in a predetermined manner on each of a front surface side and a back surface side of an insulating sheet. These coil electrodes are arranged so as to oppose each other, whereby a desired capacitance is generated. In this case, a large capacitance is obtained by making the width of the coil electrodes large.
In addition, in Japanese Unexamined Patent Application Publication No. 2001-84463, a structure is described in which a coil electrode and one opposing electrode of the capacitor are formed on the front surface side of the insulating sheet and the other opposing electrode of the capacitor is formed on the back surface side of the insulating sheet. In this structure, a conductive through hole is mechanically formed through the insulating sheet in order to connect the back-surface-side opposing electrode and a front-surface-side circuit pattern.
Furthermore, in Japanese Unexamined Patent Application Publication No. 10-334203, a coil electrode is formed on the front surface side of an insulating sheet and a coil electrode and an electrostatic-capacitance-adjusting pattern, which is for causing a capacitance to be generated, are formed on the back surface side of the insulating sheet. Then, the capacitance is adjusted by adjusting the shape (line length) of the electrostatic-capacitance-adjusting pattern.
However, with the structure of Japanese Unexamined Patent Application Publication No. 2001-84463 described above, since the coil electrode is formed to have a small number of turns and a large width, although the capacitance is large, the inductance is very small. Consequently, only a weak magnetic field can be radiated by the antenna and the distance over which communication can be performed is short. This is not suitable for data communication that requires a certain signal level.
Furthermore, in the structure of the related art of Japanese Unexamined Patent Application Publication No. 2001-84463 described above, since the insulating sheet is subjected to mechanical punching in order to physically bring the front-surface-side electrode pattern and the back-surface-side electrode pattern into conductive contact with each other, the manufacturing process is complex.
In addition, in the structure of Japanese Unexamined Patent Application Publication No. 10-334203 described above, the back-surface-side electrostatic-capacitance-adjusting pattern is formed so as to be wound in the same direction as the front-surface-side coil electrode, when viewed in plan, that is, along the direction of the magnetic field at the surface of the antenna. Therefore, the back-surface-side electrostatic-capacitance-adjusting pattern does not contribute to the inductance of the antenna and the inductance of the antenna only depends on the pattern of the front-surface-side coil electrode. Consequently, an increase in the size of the structure due to, for example, the number of windings of the front-surface-side coil electrode being increased in order to increase the inductance so as to increase the strength of the radiated magnetic field, is unavoidable.
In addition, in the case of such an antenna that includes a coil electrode, the characteristics of the antenna are determined by the inductance and capacitance of the coil electrode, which are determined by the pattern of the coil electrode. However, it is not a simple matter to design the antenna so that the inductance and the capacitance of the antenna come to have predetermined values.